1. Technical Field
The present invention is directed to a lithographic process for device fabrication and specifically to a process in which the wavelength of the exposing radiation is in the wavelength range of about 10 nm to about 350 nm.
2. Art Background
In projection lithographic processes for integrated circuit device fabrication, radiation is typically projected onto a patterned mask and the radiation transmitted through the mask (i.e. patterned radiation) is further transmitted onto a layer of an energy sensitive material formed on a substrate. Transmitting the radiation through a patterned mask thereby transfers an image of the mask pattern into the energy sensitive material. The image is then developed in the energy sensitive resist material and transferred into the underlying substrate. An integrated circuit device is fabricated using a series of such exposures to pattern different layers formed on a semiconductor substrate.
As technology advances, there is the need to provide more and more integrated circuit devices on a single chip. Consequently, the size of the individual devices on a chip are getting smaller (i.e., 0.5 .mu.m to 0.35 .mu.m to 0.25 .mu.m to 0.18 .mu.m, etc.). As the size of these devices (the device size is often referred to as the design rule) decreases, so does the wavelength of the radiation used to transfer the desired pattern into the energy sensitive material. Specifically, for design rules in the range of 0.35 .mu.m to 0.18 .mu.m, the wavelength of the exposing radiation is in the range of about 190 nm to about 350 nm (referred to as the "deep ultraviolet" or "deep-UV" range).
As noted in Ogawa, T., et al., "The effective light source optimization with the modified beam for the depth-of-focus enhancements" SPIE, Vol. 2197, pp. 19-30 (199?), one problem associated with the use of deep-UV radiation for lithography is the depth of focus of the desired image in the energy sensitive material. This is because, according to the Raleigh criterion, as the exposure wavelength gets shorter in order to obtain higher resolution, the depth of focus decreases. Ogawa et al. state that the maximum depth of focus for 0.35 .mu.m, 0.30 .mu.m, 0.25 .mu.m, and 0.22 .mu.m design rules are 1.50 .mu.m, 1.20 .mu.m, 0.80 .mu.m, and 0.63 .mu.m with 0.42, 0.48, 0.61, and 0.72 projection lens numerical aperture, respectively.
Ogawa et al. further observe that there is a limit on the achievable depth of focus using deep-UV radiation in conjunction with a conventional exposure. That limit is that an adequate depth of focus cannot be achieved for design rules smaller than 0.30 .mu.m unless super resolution techniques such as off-axis illumination are used.
Several different super-resolution techniques have been proposed to improve the depth of focus that is achieved in deep UV lithography for design rules less than 0.35 .mu.m. These techniques include off-axis illumination, phase shift masks, oblique illumination, ring illumination and quadrapole illumination. As noted by Ogawa et al., these super-resolution techniques are difficult to incorporate into a commercial process for device fabrication. Ogawa et al. propose a different technique referred to in that reference as modified beam illumination (MBI). The Ogawa et al. technique requires that the light from the radiation source (i.e., the KrF laser) be divided by a beam splitter and then overlapped again in a fly's eye lens. However, the Ogawa et al. technique requires a complex optimization process in which numerous exposures are made using a variety of light source profiles. The resulting images are then developed and analyzed to determine the light source profile that provides the desired process latitude (e.g. depth of focus) for the particular radiation source and design rules.
The difficulties of current super-resolution techniques are also described in Partlo, W. N., et al., "Depth of Focus and Resolution Enhancement for i-line and deep-UV Lithography Using Annular Illumination," SPIE: Optical/Laser Microlithography VI Vol. 1927, pp. 137-157 (1993). Partlo et al. observe that the problem associated with most of the off-axis illumination, super-resolution techniques is that they do not possess rotational symmetry. Therefore, according to Partlo et al., the off-axis super resolution techniques that do not possess complete rotational symmetry will exhibit degraded performance for some set of feature orientations. Partlo et al. describe the use of annular illumination to improve the depth of focus in deep-UV lithography because annular illumination provides complete rotational symmetry.
Accordingly, a method for improving the depth of focus and other lithographic parameters for exposures in lithographic processes for device fabrication is desired.